Due to increasing food production needs for a burgeoning global population, a significant amount of biotechnology research is being devoted to increasing the yield of crop plants. Timing of flowering can have a significant impact on production of agricultural products. For example, varieties with different flowering responses to environmental cues are necessary to adapt crops to different production regions or systems. Such a range of varieties have been developed for many crops, including wheat, corn, soybean, and strawberry. Improved methods for alteration of flowering time will facilitate the development of new, geographically adapted varieties.
Breeding programs for the development of new varieties can be limited by the seed-to-seed cycle. Thus, breeding new varieties of plants with multi-year cycles (such as biennials, e.g. carrot, or fruit trees, such as citrus) can be very slow. With respect to breeding programs, there would be a significant advantage in having commercially valuable plants that exhibit controllable and modified periods to flowering (“flowering times”). For example, accelerated flowering would shorten crop and tree breeding programs.
Improved flowering control allows more than one planting and harvest of a crop to be made within a single season. In a number of species, for example, certain grain crops, fruits, and ornamentals such as cut flowers, where the reproductive parts of the plants constitute the crop and the vegetative tissues are discarded, it would be advantageous to accelerate time to flowering. Accelerating flowering can shorten crop and tree breeding programs. Additionally, in some instances, a faster generation time would allow additional harvests of a crop to be made within a given growing season. A number of Arabidopsis genes have already been shown to accelerate flowering when constitutively expressed. These include LEAFY, APETALA1 and CONSTANS (Mandel et al., 1995; Weigel and Nilsson, 1995; Simon et al., 1996). The floral control gene LEAFY from Arabidopsis can dramatically accelerate flowering in numerous dicotyledonous plants. Constitutive expression of Arabidopsis LEAFY also caused early flowering in transgenic rice (a monocot), with a heading date that was 26-34 days earlier than that of wild-type plants. These observations indicate that floral regulatory genes from Arabidopsis are useftil tools for heading date improvement in cereal crops (He et al., 2000).
Flowering time and other developmental characteristics may be controlled by manipulating the expression of relevant transcription factors. Transcription factors can modulate gene expression, either increasing or decreasing (inducing or repressing) the rate of transcription. This modulation results in differential levels of gene expression at various developmental stages, in different tissues and cell types, and in response to different exogenous (e.g., environmental) and endogenous stimuli throughout the life cycle of the organism.
Because transcription factors are key controlling elements of biological pathways, altering the expression levels of one or more transcription factors can change entire biological pathways in an organism. This may include the alteration of development pathways in specific tissues and cell types. We have, in fact, identified closely-related CCAAT-box family transcription factors, including G3397 (SEQ ID NO: 2), G3476 (SEQ ID NO: 18) and other closely-related CCAAT-box sequences that accelerate flowering time in plants. These discoveries were made by developing numerous transformed or transgenic plant lines and analyzing the plants for an accelerated time to flower development (i.e., an “early flowering” phenotype). In so doing, we have identified important polynucleotide and polypeptide sequences for producing commercially valuable plants and crops as well as the methods for making them and using them. Other aspects and embodiments of the invention are described below and can be derived from the teachings of this disclosure as a whole.